nfsroot mount via NFS v4

[Thomas Lange]

When using a 4.X kernel FAI uses overlayfs on top of the read-only
nfsroot for making it writeable. This works only when using NFS v3.
I'm pretty sure this must be fiexed in overlayfs.

If the install client mounts the nfsroot via NFSv4 (using NFS 4.2 with
the recent 4.9 kernel) the nfsroot will remain read-only and FAI
cannot proceed.

You can force NFS v3 on the server side by adding a line to
/etc/exports as described in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=676883#10

Or you can add a nfs option into the pxelinux.cfg file when specifying
the mount address for the nfsroot:
root=1.2.3.4:/srv/fai/nfsroot:vers=3

If you want to experiment with different NFS v4 versions (using
minorversion) you can use something like this
root=1.2.3.4:/srv/fai/nfsroot:vers=4,minorversion=0

BTW, there's a new package aufs-dkms which includes aufs for 4.X
kernel. It did not test it yet. Maybe this is a solution for using
nfsroot via NFS v4 using a 4.X kernel.
--
regards Thomas